if you use a deviate, be sure to check all and everything. There are 'hidden nuggets' all over the 350 page datasheet. However, if you are going to use a devkit, at least you will avoid the hardware traps.
Re software, just approach everything as if you are dealing with not a '51, but a totally new thingy with a '51 processor inside. FORGET all you know about '51 I/O. Do not look for RCAP2, read about timer2 to see if there is a reload ability. Do not approach the IIC interface as something you have seen, apparoach it as a brand new strange animal. Do not approach the UART as something you know, apparoach it as a brand new strange animal.

If the above is not a problem for you, then the SILabs deviates are a good choice.

when working with the SILabs deviates, experience with the '51 is not an advantage, it is a hindrance

I gather you are looking for an USB slave, I am currently working with a FTDI USB master and making wondeful progress - more later.

Since you are stating "a one-off" why not just use a regular chip and a FTDI USB slave 'converter', they are cheap enough that if they save you one hour of development time, you are ahead.

Erik